1. Field of the Invention
The present invention relates to a control apparatus for a work-vehicle transmission having a transmission clutch and a gear change-speed mechanism. This control apparatus is operable, in response to an instruction for change speed, to automatically operate the transmission clutch into a non-transmitting state to operate the gear change-speed mechanism by means of an actuator and subsequently to automatically operate the transmission clutch back to a transmitting state.
2. Description of the Related Art
An example of the work-vehicle transmission to be controlled by the above-described control apparatus is disclosed by Japanese Patent Application “Kokai” No.: Hei. 6-313478. In this, a gear change-speed mechanism (denoted with a mark A in FIG. 1) to be operated by actuators (denoted with marks T1, T2 in FIG. 1) and a hydraulic multiple disc clutch (denoted with numeral 19 in FIG. 1) are arranged in series. With this, in response to a change-speed instruction from the control apparatus, the transmission clutch is automatically operated into the non-transmitting state and the gear change-speed mechanism is operated by the actuators. Upon completion of this operation of the gear change-speed mechanism by the actuators, the transmission clutch is automatically and gradually operated into the transmitting state.
A work vehicle is often subjected to a large load such as when the vehicle runs on a soft ground surface providing a significant running resistance to the vehicle or when the vehicle tows a cart mounting load therein. Hence, when the transmission clutch is operated into the non-transmitting state, the power transmission from the engine to a traveling unit of the vehicle is broken at this timing, whereby the traveling speed of the vehicle may be reduced suddenly due to the traveling load. Then, under this condition when the traveling speed of the vehicle has been reduced significantly with completion of the operation of the gear change-speed mechanism by the actuator, if the transmission clutch is operated back to the transmitting state thereby to connect the power of the engine to the traveling unit, this will result in sudden acceleration of the vehicle to the previous traveling speed before the speed reduction, thus giving a significant shock to the operator.
In this respect, according to the construction disclosed by the above document, there is provided an auxiliary transmission clutch (denoted with mark E in FIG. 1) capable of transmitting the power from the gear change-speed mechanism to the downstream side with bypassing the (main) transmission clutch. With this construction, when the transmission clutch is operated into the non-transmitting state as described above, the auxiliary transmission clutch is operated from its non-transmitting state to its transmitting state, whereby the power from the gear change-speed mechanism is transmitted via this auxiliary transmission clutch to the downstream side, thus restricting reduction in the traveling speed of the vehicle due to the traveling load. Then, when the transmission clutch is operated back to the transmitting stage after completion of the operation of the gear change-speed mechanism by the actuator, the auxiliary transmission clutch is operated from the transmitting state to the non-transmitting state. In this way, the construction functions to restrict occurrence of sudden acceleration of the vehicle to the previous high traveling speed, thus restricting occurrence of shock associated therewith.
In the case of the above-described construction disclosed by the gazette, while the operation of the gear change-speed mechanism is going on with the transmission clutch being at its non-transmitting stage, the power from the gear change-speed mechanism is transmitted to the downstream via the auxiliary transmission clutch. In this, the transmission ratio (the transmission ratio of the gear shown on the left end of a gear reduction mechanism shown in FIG. 1 of the gazette) of the power being transmitted via the auxiliary transmission clutch remains substantially fixed. With this, if a change-speed operation is effected at a higher speed range than the transmission ratio of the power transmitted via the auxiliary transmission clutch (e.g. when the transmission ratios provided by speed positions before and after the change-speed operation are higher than the transmission ratio of the power transmitted via the auxiliary transmission clutch), such change-speed operation result in change from the condition of the power being transmitted at the transmission ratio provided by the high speed position to the condition of the power being transmitted at the transmission ratio at the lower speed provided by the auxiliary transmission clutch. Hence, such operation will again cause a shock.